In both NI and ROI you will also need proof of compliance with planning permission from your architectural designer, engineer or building surveyor. This will include all aspects relating to the wastewater system and other services.
If your engineer is signing off, Engineers Ireland and other engineering institutions have standard completion certificates, if it is an architect, the RIAI/RIBA have theirs and the other professions would be affiliated to documentation from other engineering or building surveying professional bodies.
An opinion of compliance lumps certification that both planning conditions and building regulations have been abided to.
Note that one of the biggest causes of delays in getting hold of the compliance certificate (and the Building Control final certificate in NI) is supplying the certifier with all documentary evidence that has been requested.
This might include for example: ‘as-built’ energy calculations, certification of insulation products, air permeability tests, EPC & energy rating, radon barriers and other components, fire retardant paints, prefabricated structural components (including timber frame structures, roof trusses, space joists, lintel systems, pre-cast concrete, etc.), heating, ventilation, energy, electrical & security systems, fire/gas detection and alarm systems, contractors’ signed accredited details for Building Control, etc.
If you have been keeping detailed records, this part should be easy.